A digital sampled data tone control circuit for tailoring the bass response of audio signals is described by Richard J. Taylor in the British Patent Specification G.B. No. 1,385,024. This circuitry includes a low-pass filter for passing the bass spectrum of the audio signals, a multiplier coupled to the output of the low-pass filter for scaling the low-pass filtered audio signal, and combining circuitry for adding or subtracting the bass spectrum of the audio signal to the input audio signal. The combined signal represents the audio signal with the bass portion of the audio signal spectrum either boosted or cut depending on whether the low-pass filtered signal is added or subtracted from the input audio signal respectively. The degree of boost or cut is determined by the multiplication coefficient applied to the multiplier. If the low-pass filter has a fixed frequency response (that is a fixed 3 dB point) and is desirably of relatively simple design (rolls off at 6 or 12 dB per octave), adjusting the cut or boost has the undesirable affect of altering the bandwidth of the bass spectrum affected. For example, consider the low-pass filter to have a 3 dB point (a zero) of 1 kHz and a roll off of 20 dB per decade. For a bass boost or cut of 20 dB, the bandwidth of the bass spectrum affected extends to approximately 10 KHz. However, for a bass boost or cut of 1 dB, the bandwidth of the bass spectrum affected is approximately 1 KHz. To preclude such bass spectrum variations with the degree of boost or cut, the 3 dB frequency of the low-pass filter must be adjusted simultaneously with the change in multiplication coefficient. In order to satisfy this requirement, the low-pass filter will typically incorporate variable gain control elements which undesirably complicates the circuitry.
Yoshimutsu Hirata described a slightly more complicated tone control circuit for adjusting bass response at pages 77-79 of the September 1982 issue of Wireless World in the article entitled "Simple Digital Filters For Sound Reproduction". This tone control circuit includes the cascade connection of a variable finite impulse response filter and a variable infinite impulse response filter, both of which include multiplier elements. Bass boost or cut is effected by applying respective multiplying coefficients to the multiplier elements of the cascade connected filters. With proper selection of the multiplier coefficients, the bandwidth of the bass spectrum affected by boost and cut is maintained relatively constant.
It is an object of the present invention to provide a relatively versatile tone control circuit which may be conditioned to have a frequency response similar to either the Taylor apparatus or the Hirata apparatus, but which requires a single variable multiplier circuit. It is desirable to minimize the number of variable multiplier elements since they tend to be relatively complex and expensive circuit elements.